A Terahertz (THz) is a unit of frequency equal to 1012 hertz. THz electromagnetic radiation forms a large portion of the electromagnetic spectrum between the infrared and microwave regions. THz electromagnetic radiation is generally defined as covering frequencies from about 0.3 THz to about 30 THz. This corresponds to the range of wavelengths from about 1.0 mm to about 0.01 mm and is sometimes referred to as the sub-millimeter and/or far-infrared region of the electromagnetic spectrum. By way of comparison, visible light covers the frequency range of 428 THz to 750 THz, corresponding to wavelengths from 0.4 micron to 0.7 micron. Thus, THz electromagnetic radiation is in a previously much neglected range of frequencies between optical and radio waves.
THz electromagnetic radiation has interesting properties because its wavelengths are long enough to pass through some objects that are opaque to visible and IR light and are also short enough to be manipulated by conventional optical techniques. Thus, THz electromagnetic radiation can be used for the imaging of hidden objects. It is a particularly attractive means of detection because it can determine the composition, size, and shape of a variety of different substances.
The potential uses of THz electromagnetic radiation in various fields are presently being investigated. Specific applications include remote sensing, short range covert communications, compact radar ranging systems, inter-satellite communication links, testing of integrated circuits, and even medical imaging and treatment. In the field of medical imaging, for example, tumors and other pathologies may be identified and characterized. THz electromagnetic radiation may even find applications in a variety of other areas, including atmospheric sensing and upper atmospheric imagery.
Research into the potential use of THz electromagnetic radiation for the detection and characterization of contraband, such as chemicals (including illegal drugs, explosives, and toxic substances), biological agents (including aerosols) and concealed weapons, is being stimulated by law enforcement and homeland security concerns. By using THz electromagnetic radiation, drugs, explosives, and pathogens can be identified in parcels before they are opened. Concealed guns and knives can be observed on people. Even mines in minefields can potentially be located.
Further, the use of ambient THz electromagnetic radiation facilitates standoff detection of weapons and explosives. Standoff detection is particularly useful in determining whether a threat exists prior to the item or person posing the threat actually entering a sensitive area. That is, the threat can be identified before there is an opportunity to do the intended harm. For example, a THz electromagnetic radiation imaging system can be employed at the gate to a military base. If a threat is perceived, then the person posing the potential threat can be denied entry.
THz time-domain spectroscopy and related THz technologies promise to be of great benefit for military and civilian uses, because they offer innovative imaging and sensing technologies that can provide information not available through such conventional methods as microwave and x-ray. Spectral fingerprints can facilitate the identification of suspicious items. The resolution of such spectra can be impressive. Not only can the type of explosive, drug, or pathogen be determined, but many times the factory where the substance was manufactured can be identified, as well. Thus, the potential forensic benefits are clear.
Compared to the relatively well-understood science and technology at microwave and optical frequencies, THz science and technology is in its infancy. This is largely due to the inadequate power of available contemporary THz sources and the limited resolution of contemporary THz imaging receivers.
The use of THz electromagnetic radiation in such applications is particularly attractive because, in many instance, it can be used passively. That is, THz electromagnetic radiation can be viewed without actively illuminating the subject. Other forms of imaging, such as those using x-ray and gamma ray backscatter, require that the subject be illuminated so as to provide the necessary radiation. Such illumination tends to pose some degree of health risks to human subjects and imaging equipment operators. However, in many instances sufficient THz electromagnetic radiation is generally present in the ambient environment to facilitate imaging. Thus, illumination is not always necessary for THz imaging. The elimination of a source of radiation by using ambient radiation both reduces costs and eliminates health concerns. It is also useful in covert applications.
In industrial applications, such as manufacturing and quality control, the use of an active source may be less objectionable. In such settings, provision for shielding can more readily be made. Further, in industrial settings the cost and space required for a source tend to be less problematic. Imaging can potentially be accomplished in either a transmission mode or a reflection mode of operation. In the transmission mode, the radiation source is on the opposite side of the subject with respect to the detection device and the radiation is transmitted through the subject to detect substances therein. In the reflection mode, the radiation source is on the same side of the subject with respect to the detection device and the radiation is reflected by substances within the subject.
Sensors for detecting THz electromagnetic radiation are presently being developed. Although prices are expect to decrease as development proceeds and as the sensors are produced in volume, the cost of contemporary sensors is high.
Another problem associated with the use of THz electromagnetic radiation is that contemporary THz electromagnetic radiation sources with suitable output power levels are undesirably large, heavy, and costly. The only way to generate THz electromagnetic radiation with average power over a watt, according to contemporary practice, is to use an accelerator, such as a 15 MeV synchrotron. As those skilled in the art will appreciate, such THz electromagnetic radiation sources are very large and very expensive. Even in industrial environments, it is generally desirable to reduce the size and cost of equipment.
In view of the foregoing, it is desirable to provide a source of THz electromagnetic radiation with output power levels of a few hundred milliwatts and higher that is smaller in size and less costly than contemporary sources. It is also desirable to provide a way to more readily facilitate the imaging in the THz frequency range of the electromagnetic spectrum.